I. Field of the Invention
This invention relates to an organic thin film display element.
II. Description of the Prior Art
In recent years, the development of the technique of forming an extremely thin film of organic material represented by Langmuir-Blodgett process (hereinafter referred to as "LB process") has lead to the active application of said technique to various electronic elements.
Studies in this particular field (represented, for example, by the research work on the MIS type semiconductor element using a thin organic film) are widely pursued. Under the present state, however, no new functional element has been practically proposed which effectively utilizes the properties peculiar to said thin organic film.
Among the properties of the organic material, that which draws the greatest attention from the view point of applying said material to a functional element is a charge transfer phenomenon among the molecules. In this connection it will be noted that some of the organic materials comprise donor molecules which have a low ionization potential and tend to be converted into positive ions by supplying electrons to other molecules. The other organic materials are formed of acceptor molecules which have a high electron affinity and receive electrons from other molecules and tend to be negative ions by themselves. It is well known that these two types of molecules are combined into a compound generally referred to as a charge-transfer complex. For instance a complex of perylene and tetracyanoquinodimethane (TCNQ) is formed of neutral molecules giving rise no charge transfer. In contrast, a complex of tetramethylphenylenediamine (TMPD) and TCNQ represents an ionic compound composed of positive and negative molecules. It is also known that a compound of tetrathiofulvalene (TTE) and TCNQ indicates a transition from a neutral to an ionic state depending on temperature and pressure.
When the charge transfer phenomenon of the above-mentioned organic material is utilized as an operation principle of an element, it is required that the organic material possess of excellent charge transfer properties including quick response and high controllability, and further can be easily prepared. However, the satisfaction of the above-mentioned requirements is accompanied with the drawbacks that the charge-transfer complex crystal presents tremendous difficulties in the growth of a crystal, and it is difficult to externally control the charge transfer. Further, the control of a charge transfer between a metal thin film and organic thin film by means of a light or electric field is applied to a switching element or memory element. However, this attempt is accompanied with great drawbacks in respect of the charge transfer efficiency, the speed of responsibility, the effective life of the processed elements, etc. As mentioned above, the phenomenon of charge transfer in an organic material has not yet been fully put to practical application, though great hope has been pinned on the utilization of the charge transfer phenomenon to the organic element.
Another noticeable property of an organic material in question is its optical characteristic. Like various organic coloring materials, many of the proposed organic materials respond to light falling within the visible range and some are possessed of a high light-absorbing property. To date, an electro-chromic display (ECD) element has been known as a element to which the optical property of the aforementioned organic material has been applied. This ECD element is characterized in that the ionic condition of an organic material is changed by impressing an electric field an organic molecules in a solution, and the display of an image is carried out by changes in the absorption spectrum occurring in the visible range. The ECD element offers the advantages that an image is distinctly displayed and the view field is very much broadened.
Nevertheless, the ECD element is still accompanied with the drawbacks that the behavior of the ECD is followed by the shifting of ions or protons, resulting in an extreme decline in the responding speed and operating efficiency. Moreover, the ECD element presents difficulties in the repeating characteristic and moreover has a short effective life. Consequently, the ECD element is retarded in practical application in spite of its excellent optical properties.
Further, there is a great demand for the development of a new element which is possessed of such a novel function as can not be expected from the conventional electroluminescence device, light-emitting diode (LED), liquid crystal display element, etc. Such a novel function may be represented, for example, by that of a single picture element capable of displaying various tints. A light-emitting display element represented by the conventional LED has the drawback that its light-emitting property is defined by the parameter of the raw material of said element. Tremendous difficulties are encountered in controlling the wavelength of an emitted light by an external factor such as voltage. Moreover, a liquid crystal display element indicates colors by means of filters of red, green and blue colors. Essentially, therefore, one picture element of such an element is capable of displaying only one color.